Recently, the study on silicon photonics in which various optical devices are integrated on a silicon substrate has received extensive attention.
It is known that silicon is an indirect transition type material, and thus a light source with high energy efficiency is difficult to be realized using the silicon. On the other hand, a compound semiconductor such as GaAs (gallium arsenide) and InP (indium phosphide) is a direct transition type material, and thus a light source with high energy efficiency is able to be realized using the compound semiconductor. For this reason, as a method for integrating the light source on the silicon substrate, a method for integrating a compound semiconductor light source on the silicon substrate is considered as a superior method.
For example, as illustrated in FIG. 28, when a compound semiconductor laser diode is bonded on the silicon substrate as the light source by flip-chip bonding or the like, it is possible to integrate the light source with high energy efficiency on the silicon substrate (Non Patent Document 1).
Light from the compound semiconductor laser diode arranged as described above, for example, is delivered to an optical integrated circuit through a spot size converter, a silicon optical waveguide, or the like. In addition, in order to deliver the light from the compound semiconductor laser diode to a plurality of optical devices on the silicon substrate, a light source circuit including a branched optical waveguide as disclosed in Non Patent Document 2 is used.
As a representative example of the branched optical waveguide, a 1×2 multimode interference waveguide is illustrated in FIG. 29. The 1×2 multimode interference waveguide branches one input light into two output terminals. As illustrated in FIG. 30, when the one light ray is branched into a plurality of light rays by using the light source circuit including the branched optical waveguide, the light from the compound semiconductor laser diode is able to be effectively utilized. As the branched optical waveguide, it is preferable that a multi-stage branched optical waveguide with which the light is able to be distributed to a plurality of optical devices be used.
When the light from the light source such as the semiconductor laser diode is incident on the branched optical waveguide, reflection light is generated in a branch section. As the light source device illustrated in FIG. 30, when the semiconductor laser diode and the branched optical waveguide are combined to be operated, returning light with respect to the laser diode may be generated in the branched optical waveguide. When the returning light is incident on the laser diode, a variation in output intensity of the laser diode (that is, generation of an intensity noise) or a variation in emission wavelength (generation of a phase noise) is caused, and thus a problem of affecting an oscillation property occurs.
Thus, in the light source device of the related art using the semiconductor laser diode and the branched optical waveguide, a problem in which operation of the light source device is unstable due to the returning light from the branched optical waveguide exists.
In addition, as described above, it is preferable that the multi-stage branched optical waveguide be used as the branched optical waveguide such that the light is able to be distributed to the plurality of optical devices. However, in the multi-stage branched optical waveguide, when the number of stages of the branch section is increased in order to increase the number of distributed light rays, the reflection light is generated in the branch section of each stage, and thus a problem in which the intensity of the returning light is likely to be high occurs.
As one of methods for inhibiting the returning light, a method in which an optical isolator which uses a nonreciprocal phase shift effect according to a magneto-optical material (an effect in which a phase change amount received by the light is different according to a propagation direction) is used is suggested (Non Patent Document 3). In this method, an advantage by which the returning light is able to be effectively inhibited exists. However, it is necessary that a magnetic field be applied to the magneto-optical material in order to realize operation of the optical isolator, and thus a problem in which a device for generating a magnetic field has to be additionally disposed exists. In addition, since the magneto-optical material has a physical property which is substantially different from that of silicon, a problem in which the magneto-optical material is difficult to be integrated on the silicon substrate exists.
In addition, in order to solve the problem due to the returning light, the reflection light generated in the branched optical waveguide may be inhibited. As a method for reducing the reflection light generated in the branched optical waveguide, various methods are proposed.
For example, as disclosed in Patent Document 1 and Patent Document 2, a branched optical waveguide including a branch section of a special shape is proposed. However, even though the branched optical waveguide generates little reflection light in a theoretical design, a problem in which the reflection light according to a manufacturing error is not able to be prevented from being generated exists. In this case, it is possible to reduce the reflection light with an improvement of microfabrication precision, but enormous cost and rigorous process management are required in order to improve the microfabrication precision.
In addition, in the light source circuit as described above, a branched optical waveguide using silicon is commonly used as the branched optical waveguide because there is an advantage of enabling substantial downsizing compared to a branched optical waveguide using a quartz-based material. However, the branched optical waveguide using silicon has a critically large core-cladding refractive index difference, and thus has a drawback in which the reflection light is difficult to be reduced.